1. Field of the Invention
The present invention relates to a shift controller for a continuously variable transmission. More specifically, the present invention relates to a shift controller for a continuously variable transmission including a shift actuator that changes a gear ratio regardless of engine speed.
2. Description of the Background Art
There are many known hybrid vehicles. An electric vehicle powered by a motor offers the advantage of no air pollution, reduced noise pollution, and better response to acceleration and deceleration needs as compared with conventional vehicles powered by engines. A hybrid vehicle mounted with a motor and an engine has been put into practical use as an embodiment having these benefits.
Three types of such a hybrid vehicle are generally known: a “series hybrid system,” a “parallel hybrid system,” and a “series-parallel combined system.” Specifically, the series hybrid system is powered solely by the motor, with the engine being used for generating electricity for recharging a battery. The parallel hybrid system uses both the motor and the engine to drive the vehicle, each being used according to a running condition and the like. The series-parallel combined system uses the foregoing two systems, one being selected for use appropriately according to the running condition.
In many of these hybrid vehicles, a belt-type continuously variable transmission is adopted as an automatic transmission. The belt-type continuously variable transmission includes a driving side pulley, a driven side pulley, and an endless belt. The driving side pulley is connected to an output shaft of a power source. The driven side pulley is connected to a driving shaft. The endless belt is wound around the driving side pulley and the driven side pulley. The gear ratio is changed by displacing a radius of the driving side pulley using a centrifugal force generated by rotation of the output shaft.
Japanese Patent Laid-open No. 2004-116672 discloses an electronically controlled belt-type continuously variable transmission in the place of a conventional belt-type continuously variable transmission. The electronic controlled belt-type continuously variable transmission includes a separate shift actuator that displaces the radius of the driving side pulley called an electronic belt converter. The electronic belt converter is capable of arbitrarily controlling its gear ratio regardless of the speed of the output shaft.
FIG. 9 is a diagram showing a typical shift pattern of a conventional electronic belt converter. The relation among an engine speed Ne, a vehicle speed V, and a gear ratio R of the continuously variable transmission has been previously registered. The shift pattern includes a low ratio control range, a top ratio control range, and a shift control range. In the low ratio control range, the engine speed Ne is variably controlled at a low speed range with the gear ratio R set at a predetermined low ratio Rlow. In the top ratio control range, the engine speed Ne is variably controlled at a high-speed range with the gear ratio R set at a predetermined top ratio Rtop. In the shift control range, the gear ratio is variably controlled with the engine speed Ne fixed at a boundary between the low speed range and the high-speed range.
There is known a system, in which power of an engine is used to drive a generator for generating electricity which, in turn, is used to charge a battery. In such a system, the more a charging current, as a result of an amount of charge still available for use in the battery or a remaining charge of the battery, the greater a driving torque for the generator. This results in the engine mechanical load increasing. Thus, a rider is required to operate the vehicle with a relatively open throttle. Therefore, to obtain running performance equivalent to that associated with a sufficient remaining charge of the battery when the remaining charge of the battery is low, a rider needs to operate the vehicle with the throttle even more open. This gives the rider an impression different from that during ordinary operation. Consequently, there is still a need for a control apparatus for a continuously variable transmission that does not give the operator the above-mentioned different impression.